The moon orbits around the Earth, and goes through its cycle of phases, in a period of time slightly less than a month; indeed, the very word "month" comes from the orbital cycle of the moon. If the moon's orbit around Earth were in the same plane as the Earth's orbit around the sun, then every month at new moon the moon would pass directly between the Earth and the sun and some place on Earth would experience a solar eclipse. However, since the moon's orbit is tilted some five degrees with respect to the Earth's orbit, solar eclipses can occur only when new moon takes place near the same time that the moon is crossing the plane of the Earth's orbit; such "eclipse seasons" take place at approximately six-month intervals. During such an "eclipse season" there may also be a lunar eclipse, i.e., the moon will pass through at least part of the Earth's shadow, during either the preceding or following full moon.

We are going through an "eclipse season" right now. The solar eclipse took place two weeks ago, on Sept. 13; this was a partial eclipse that was visible from far southern Africa, the southern Indian Ocean, and parts of Antarctica. Now, we are approaching full moon, which takes place on this coming Sunday evening, Sept. 27, and accordingly we have a lunar eclipse; this is a total eclipse, wherein the moon crosses completely into the Earth's shadow.

By the time the moon rises as seen from here in southern New Mexico it will already be passing through the Earth's outer shadow, or "penumbra." At 7:07 p.m. it starts entering the Earth's dark inner shadow, or "umbra," commencing the partial phase of the eclipse. The total eclipse begins at 8:11 P.M. when the moon is fully immersed within the umbra, and lasts until 9:23 p.m., when the moon starts exiting the umbra. The moon fully leaves the umbra at 10:27 p.m., and meanwhile the hazy shading of the penumbra may remain visible for another half-hour or so after that.

Even when fully immersed within the umbra the moon will not entirely disappear. The Earth's atmosphere scatters and bends, or "refracts," sunlight, and because red light is scattered the least of any color that our eyes detect, the moon generally takes on a reddish to orange-tinted coloration during a total eclipse. (This is the same reason that sunrises and sunsets appear reddish and orange-tinted; in effect, we are seeing the reflections of all the sunrises and sunsets taking place on Earth at the time of the eclipse.) If there is an unusual amount of dust in the atmosphere due to a recent volcanic eruption the moon's coloration could be significantly darker, but since there haven't been any such eruptions lately this is unlikely to take place during Sunday's eclipse.

There are several things associated with this eclipse that make it somewhat unique. The (northern hemisphere's) autumn equinox was just this past Thursday (September 23), and thus this full moon is traditionally known as the "Harvest Moon." While the average time between successive moonrises from night to night is about 50 minutes, because of the angle that the moon's orbit makes with respect to the horizon this time of year, this difference is reduced to about 20 minutes around the time of full moon. The name accordingly comes from the fact that this phenomenon occurs near the traditional time of harvest.

The moon's orbit around Earth is not a perfect circle, but rather is an ellipse. It so happens that the moon is at its closest point to Earth, its "perigee," just an hour before the middle of the eclipse on Sunday. While this would be difficult to detect with the unaided eye to anyone who was unaware of the fact, the apparent size of the moon will be some 13% larger than it appears when it is at its farthest point from Earth, i.e., its "apogee."

One more thing about Sunday's eclipse that makes it rather unique is that it is the fourth, and last, eclipse of a "tetrad," i.e., four consecutive total eclipses during four consecutive "eclipse seasons." The current tetrad started with an eclipse back in April 2014, with another one last October and the third onetaking place this past April 4. Tetrads are relatively uncommon; the previous one took place in 2003-04, and the next one does not occur unti 2032-33. This current tetrad is perhaps even more unusual in one sense in that all four eclipses, in theory, anyway, have been visible from New Mexico, although last April's eclipse was clouded out (at least, from this author's residence).

There will continue to be lunar eclipses during the forthcoming "eclipse seasons," but these will be relatively unspectacular events, indeed, the next three are "penumbral" eclipses, wherein the moon only passes through the penumbra and, at best, only exhibits a slightly hazy shading. The eclipse on the morning of March 23, 2016 is, theoretically, visible from New Mexico before dawn, and the one on the evening of February 10, 2017 (wherein the moon will be completely immersed within the penumbra) should also be detectable from New Mexico during dusk.

Not until Jan. 31, 2018, will there be another total lunar eclipse. That one will also be visible from New Mexico, with totality taking place during the late morning hours and dawn, and ending right around the time the moon sets and the sun rises.

Alan Hale is a professional astronomer who resides in Cloudcroft. He is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His web site is http://www.earthriseinstitute.org.